Estrogen and the estrogen receptors (ERs) are critical regulators of breast epithelial cell proliferation, differentiation, and apoptosis. Consequently, ER-positive breast cancers are frequently treated with hormonal therapies designed to inhibit receptor function (antiestrogens) or block production of estrogens (aromatase inhibitors). While these treatments have proven to be extremely effective in the clinical setting, both de novo and acquired resistance to these drugs are a significant problem. Most concerning is the fact that treatment failure often correlates with disease progression and metastasis. This proposal seeks to understand the regulation and function of a signaling axis that is hypothesized to function as a mediator of these aggressive breast tumor phenotypes. It focuses on a triad of signaling molecules that includes the protein tyrosine kinase c-Src, the adapter molecule p130Cas (Cas), and a cytoplasmic molecule called breast cancer antiestrogen resistance-3 (BCAR3). Each of these is highly expressed in breast cancer cell lines arising from advanced cancers and, in those cases where expression in human cancers has been measured, aggressive breast tumors. We hypothesize that these proteins promote resistance to endocrine therapies and disease progression through up-regulation of proliferative, survival, and cell migration pathways and down-regulation of apoptotic pathways. The studies outlined in Aims 1 and 2 of this proposal will define the molecular composition and regulation of pathways activated by Cas, c-Src, and BCAR3 that control antiestrogen responses and cell migration/invasion of breast tumor cells. We will then turn in Aim 3 to mouse xenograft models to investigate whether inhibition of the Cas-Src-BCAR3 pathway can re-sensitize resistant ER-positive breast tumors to antiestrogen therapies and inhibit the growth and metastasis of ER-negative tumors. The long-term goal of this project is to gain an understanding of the regulation, function, and molecular composition of Cas-Src-BCAR3 signaling networks in order to be able to 1) better predict which tumors will have a high likelihood of exhibiting resistance; 2) develop therapeutic approaches for re-sensitizing resistant tumors to these classes of drugs; 3) design combinatorial therapies that circumvent the resistance to single-agent hormona treatment; and 4) develop novel strategies for limiting metastatic spread of breast tumors. PUBLIC HEALTH RELEVANCE: This study focuses on the Cas-Src-BCAR3 signaling axis, which is hypothesized to play a central role in regulating global responses that contribute to aggressive tumor phenotypes and resistance to estrogen- targeted therapies in ER-positive cells. However, these effects are not restricted to ER-positive cells; breast tumor cells in which this pathway is highly active also exhibit resistance to cytotoxic therapies and increased migration/invasion, irrespective of ER status. By better understanding the composition, function, and regulation of Cas, Src, and BCAR3 signaling networks, it may be possible to 1) better define molecular signatures that will predict which tumors will fail endocrine therapies; 2) develop targeted treatments to block the specific pathways that are deregulated, and 3) consider the use of hormonally- targeted therapies in combination with those targeting the deregulated pathway(s) to provide maximum efficacy of treatment for breast cancer.